Liquid gel coat on-mold coating is a known technique for decorating or protecting the surface of a molded article formed from thermosetting resins, whether or not reinforced with glass fibers. In this technique, a liquid gel coat, which becomes the outer surface or skin of the molded article, is sprayed onto the interior wall of a female mold prior to molding the part. After the gel coat layer has hardened sufficiently, one or more liquid thermosetting fill resin layers, with or without glass fiber reinforcement, which constitute the bulk of the finsihed article, are then laid up or sprayed up over the gel coat. Layers are added and allowed to cure as needed to build the article to the desired thickness. After the cure has advanced sufficiently and the gel coat and fill resin layers are integral, the finished coated article is released from the mold which is later reused.
On-mold coating as described above is distinguished from post-mold coating processes, in which the fill resin is molded before the coating is introduced in the mold, and from conventional decorating operations, in which the fill resin is molded and cured in the mold, then released from the mold and decorated with a coating powder or other finish. On-mold coating is also distinguished from in-mold coating processes, in which matching molds are utilized and the coating and fill resin are cured together in a closed molding environment under heat and pressure.
There are a number of drawbacks associated with the use of liquid gel coats during on-mold coating. For instance, liquid gel coats are hard to apply uniformly to the mold surface and overspray must be collected and removed as hazardous waste. Consequently, the transfer efficiency of liquid gel coats is extremely poor (i.e., about 38%). Liquid gel coats also contain alarmingly high levels of volatile organic solvents or crosslinking liquid monomers, such as liquid styrene monomers, which tend to flash away when sprayed on the mold, thus changing the coating formulation, creating bubbles, undesired porosity, and other irregularities in the surface coating, and generating VOC's at unsafe levels, making it necessary to contain and collect the vapor of volatile ingredients. Also, this manner of operation results in long cycle times as the gel coat must be allowed to harden for several hours before application of the fill resin. Lastly, the resulting surface coating, despite being very thick is insufficiently resistant to scratching, cracking, impact, light, heat, moisture, salinity, weathering and solvents.
In view of the foregoing drawbacks, recent emphasis has been placed on finding a suitable replacement for liquid gel coats. High solids and water-borne liquid coatings have been tried, but they fail to deliver the needed performance. Thermosetting coating powders have also been proposed. Coating powders have a number of advantages over liquid gel coats. For instance, they are essentially free of volatile organic solvents, and, as a result, give off little, if any, VOC's to the environment when cured. In addition, coating powders improve working hygiene, as they are in dry, free-flowing, solid form and have no messy liquids associated with them to adhere to workers' clothes and coating equipment. They are relatively non-toxic and in the event of a spill are easily swept up without requiring special cleaning and spill containment supplies. Lastly, oversprayed powders can be recycled during the coating operation and recombined with the original powder feed, leading to very high (i.e., almost 100%) transfer efficiencies and minimal waste generation.
However, thermosetting coating powders are not without problems. Traditionally, they have not been suited for application onto heat sensitive substrates, including plastic molds, such as the unsaturated polyester molds normally employed in the manufacture of molded articles described above, due to the rather high temperatures demanded to melt-flow and cure the powders. Because such molds are rather expensive and must be reused over and over again, thermal damage caused by curing at temperatures above their softening point or plastic deformation temperature cannot be tolerated. While a number of lower temperature curing thermosetting coating powders based on unsaturated polyester resins have been proposed for on-molding coating purposes, they also have suffered from significant drawbacks, such as an inability to sufficiently cure on the surface in an open air molding process, making such powders useful only in a closed molding environment, or an inability to resist blocking or sintering at room temperature, rendering such powders physically unstable and virtually unusable after prolonged storage.
U.S. Pat. No. 4,316,869 (Van Gasse) teaches a method for on-mold coating of molded articles, particularly fiberglass-reinforced boat hulls, with thermosetting coating powders. Specifically disclosed are powdered unsaturated polyester resin formulations containing an unsaturated polyester resin, a copolymerizable cross-linking diallyl ester prepolymer, a cure initiator, along with other common additives. Also required therein is a high-boiling, copolymerizable cross-linking monomer, in particular di- or tri-functional allyl-containing monomers, such as triallyl cyanurate and triallyl isocyanurate. Yet, there are disadvantages to using cross-linking monomers. For instance, such monomers are typically liquids or waxy (low melting) solids at room temperature which have only limited use in coating powders. When employed beyond trace amounts, they tend to dramatically lower the glass transition temperature (Tg) of the formulation, causing the powders to block or sinter during storage and making them virtually impossible to meter and spray during commercial coating operations. Conversion of such materials into higher melting solids is rather expensive and time-consuming.